This invention relates to an anchor bolt construction, and more particularly to an expansion sleeve part of an anchor bolt, and to a method of manufacture of an anchor bolt.
It is common in the prior art for anchor bolts to be embedded in a mounting surface, such as a solidified concrete surface (hereafter referred to simply as a concrete surface) as a means of mounting objects onto the concrete surface.
A conventional anchor bolt generally consists of a bolt body 1 and a sleeve 2 as shown in FIGS. 1 and 2. A hole 4 is drilled into the concrete surface and an enlarged part is undercut at the bottom of the hole, the enlarged part having a flared or conical surface. In order to allow the bolt body 1 to be embedded into the hole 4, the lower end or head 1a of the bolt body 1 is conical, and the upper end has a spiraling thread 1b for securing an object which is to be secured to the concrete surface.
In order to allow the bolt body 1 to pass through the hole 4 in the concrete surface when embedding it into the concrete surface, the maximum diameter of the tapered head 1a of the bolt body 1 is the same as or slightly smaller than the diameter of the upper non-flared part of the hole 4 in the concrete surface. As a result, there initially is a gap between the tapered head 1a of the bolt body 1 and the inner wall of the flared part of the hole 4. Then, the sleeve 2 is expanded into this gap in order to secure the bolt body 1 in the hole 4.
The sleeve 2 encloses the lateral circumference of the bolt body 1 and it is interposed between the flared part of the hole 4 and the head 1a. At the lower end of the sleeve 2, a plurality of slits 2a are formed in the axial direction and extend to the bottom of the sleeve. When this sleeve is installed, the lower end of the sleeve is expanded out to anchor the bolt body 1 in the flared part of the hole 4, as shown in FIG. 1.
Up until now, as shown in FIGS. 1 and 2, the sleeve 2 of the anchor bolt has been composed of a one-piece cylinder for the entire length of the sleeve 2. The slits 2a which open to the bottom end of the sleeve have been formed in the lower end of this cylinder in the axial direction, thus providing, at the lower end of the sleeve 2, expansion sleeve pieces 2c between the slits which are capable of flaring out freely. Thus, as shown in FIGS. 4 and 5, the radius of curvature R1' (refers to the curvature in the transverse direction; the same holds true hereafter) of the inner walls of the expansion sleeve pieces 2c has been the same as the radius of curvature R3 (FIG. 3) of the upper parts of the sleeve 2 where the slits 2a were not formed (in other words, the curvature of the inner wall of the cylinder).
For this reason, when the installation of the anchor bolt is complete after expanding the pieces 2c as shown in FIGS. 1 and 5, the radius of curvature R1' of the inner peripheral wall of the expansion sleeve pieces 2c has been considerably smaller than the radius of curvature R2' of the outer peripheral wall of the tapered head la at the lower end of the bolt body (see FIG. 5). As a result, the anchoring of the head 1a of the anchor bolt 1, when it is anchored in the hole, has been unstable.
In other words, if a powerful force were to act upon the anchor bolt 1 in either the lateral direction or the axial direction, because the expansion sleeve 2 would tend to deform outwardly as indicated by the arrows 18 in FIG. 5, the radii of curvature of the outer and inner walls of the expansion sleeve would tend to widen, as shown in FIG. 6, so as to approach or become the same as the radius of curvature R5 of the inner peripheral wall of the flared part of the hole 4; or the radius of curvature R2' of the outer peripheral wall of the tapered head at the lower end of the bolt body.
Thus, when the curvature of the lower expansion part of the sleeve widened, a gap 19 (see FIG. 6) would appear between the anchor bolt and the hole in which it was embedded, making it impossible to maintain a strong and stable anchorage of the anchor bolt in the hole.
In addition, with the anchor bolt sleeves used up until now, because the lower expansion part and the upper cylindrical part were constructed in a single piece, internal stress from the upper cylindrical part, which would resist expansion, would act upon the intermediate connecting portion between the parts, thus making it difficult for that connecting portion to expand during installation. As a result, when the anchor bolt was embedded, this intermediate connecting portion would sometimes not be in perfect contact with the outer peripheral wall of the bolt body, which would then cause looseness in the anchor bolt after it had been embedded in the mounting surface.
It is a primary object of this invention to provide an anchor bolt expansion sleeve and a manufacturing method for such a sleeve, which avoids the problems described above and which can be constructed at low cost.